This invention relates generally to digital memory systems and more particularly to digital memory systems which are adapted to store radio frequency signals and to enable subsequent retransmission of such signals.
As is known in the art, it is frequently desired to store a received radio frequency signal and later retransmit such signal. In one such system the received radio frequency signal is periodically sampled at or above the Nyquist frequency, each sample is next converted into a corresponding digital word and each digital word is then stored in a digital memory. When it is desired to retransmit, the stored digital words are sequentially read from the memory in the sequence in which they were stored and such digital words are converted into corresponding voltages to produce a radio frequency signal which is amplified and retransmitted.
With such arrangement the degree to which the retransmitted radio frequency signal resembles the received radio frequency signal is related, inter alia, to the degree of quantization of the samples, i.e. the number of bits in each digital word. On the other hand, the cost, weight and size of the memory system increases as the degree of quantization increases. In particular, as the number of bits representing each sample is reduced the retransmitted radio frequency signal will, as a result of the quantization error, include, in addition to a radio frequency signal having the frequency of the received signal, unwanted harmonics or spurious signals which distort and reduce the amount of available power which may be used to amplify a radio frequency signal having the frequency of the received signal. For example, if one bit quantization is used the received radio frequency signal (within the predetermined band of frequencies f.sub.a -f.sub.b) is sampled at or below the Nyquist frequency (i.e. a frequency equal to or greater than 2f.sub.b) and is converted from a sinusoidal signal to a square wave signal having a repetition frequency equal to the frequency of the received signal, say a frequency f.sub.1, where f.sub.b .gtoreq.f.sub.1 .gtoreq.f.sub.a. As is known, while the frequency spectrum of an "unsampled" square wave includes a frequency component at the repetition frequency f.sub.1, such spectrum also includes odd harmonics 3f.sub.1, 5f.sub.1, . . . etc.; however, the frequency spectrum of a "sampled" square wave signal includes the frequency spectrum of an "unsampled" square wave signal "folded" repeatedly about harmonics of the sampling frequency. Therefore, because the sampling frequency 2f.sub.b may be less than twice the frequency of, say, the third or fifth harmonic of the square wave signal (i.e. 6f.sub.1, 10f.sub.1), the frequency spectrum of the sampled square wave signal may include, in addition to a frequency component f.sub.1, unwanted harmonics produced by the "folding". As described above, these unwanted harmonic components distort and reduce the amount of available fundamental frequency power which may be used to amplify a radio frequency signal having the frequency of the received signal.